1. Field of the Invention
The present invention relates to a cleaning composition for contact lenses. More particularly, it relates to a cleaning composition particularly useful for water-absorptive contact lenses.
2. Description of the Prior Art
Heretofore, various kinds of contact lenses have been used. As typical contact lenses, there are water-nonabsorptive contact lenses made of polymethyl methacrylate or silicone rubber and water-absorptive contact lenses made predominantly of poly-2-hydroxyethyl methacrylate or polyvinyl pyrrolidone.
When these contact lenses are worn on eyes, secretions in the eyes, such as proteins or sebum, tend to deposit on the surfaces of the lenses. Therefore, upon removal from the eyes, the contact lenses must be cleaned to remove the deposited proteins or sebum from the lens surfaces. If inadequately cleaned contact lenses are inserted into eyes, they are likely to cause uncomfortable symptons such as foggy sight, pain or ocular injection, and in an extreme case, it will be necessary to stop wearing the contact lenses.
In a method commonly used for cleaning contact lenses, a cleaning solution comprising e.g. a higher alcohol ether is applied to both surfaces of the lenses and the lens surfaces are then rubbed with fingers. The cleaned contact lenses are rinsed with e.g. a physiological saline solution and then stored in a preservative solution comprising e.g. sodium chloride and a buffer.
Particularly in the case of water-absorptive contact lenses, the cleaning solution is likely to penetrate into the lenses during the cleaning operation because they are made of water absorptive material. Accordingly, it is likely that the cleaning solution remains absorbed in the lenses if the subsequent rinsing is inadequate, or some ingredients of the cleaning solution still remain in the lenses even after the rinsing. In view of the likeliness that such contact lenses as containing the cleaning solution will be inserted to eyes, it is necessary that the cleaning solution is made of ingredients which are harmless or safe to the ocular tissues.
Accordingly, the surface active agent to be used for the cleaning solution, should be selected from those having an adequate cleaning power and being harmless to the ocular tissues and stable.
In this respect, a block copolymer prepared by the addition of ethyleneoxide to both terminals of poly-propylene oxide, has been commonly used as the surface active agent. This block copolymer has desirable characteristics such that when used as a cleaning solution for water-absorptive contact lenses, it has little toxicity and is harmless or quite safe to the ocular tissues and by virtue of its great molecular weight, it is hardly penetrable into the contact lenses during the cleaning operation of the water-absorptive contact lenses.
However, when the ethyleneoxide content and the molecular weight of the block copolymer are adjusted to increase its cleaning power, the cloud point of the cleaning solution (i.e. the temperature at which the cleaning solution becomes turbit) tends to be low, or in some cases, the cloud point lowers as time passes due to the effect of light (i.e. ultraviolet ray), whereby the cleaning solution becomes turbid at a temperature of about 50 to about 70.degree. C. Consequently, the cleaning power will be reduced, the stability will be lost and the above mentioned effectiveness can no longer be expected.